Tanning lamps are similar to conventional mercury-vapor fluorescent lamps in construction except that they contain one or more phosphors which emit radiation in the ultraviolet (UV) region of the electromagnetic spectrum in order to induce a tanning response in human skin. The UV emission from tanning lamps is classified into two regions, UVA and UVB. The U.S. Food & Drug Administration (FDA) defines the UVA region as being from 320–400 nm and the UVB region as being from 260–320 nm. (See, Performance Standards for Light-Emitting Products: Sunlamp Products and Ultraviolet Lamps intended for use in Sunlamp Products, 21 C.F.R. 1040.20) The majority of the UV flux of tanning lamps is emitted in the UVA region with the balance in the UVB region. This is intended to generally mimic the relative proportions of UVA and UVB in natural sunlight. Both UVA and UVB have a role in the tanning process. When human skin is exposed to UV light, the melanocytes in the skin's epidermis release extra melanin, a protein pigment responsible for tanning. While UVB radiation is more efficient than UVA in stimulating the production of melanin, UVA radiation is much more efficient than UVB in oxidizing the melanin to produce the brown color which is characteristic of suntans.
The typical UV-emitting phosphor employed in tanning lamps is a single-component lead-activated barium disilicate, BaSi2O5:Pb. The BaSi2O5:Pb phosphor intrinsically has low maintenance due to its affinity for mercury. Hence, when this phosphor is used in a mercury-vapor fluorescent lamp, the phosphor surface is usually protected by a coating, typically aluminum oxide, as described in U.S. Pat. Nos. 4,585,673, 4,710,674, 4,825,124 and 5,523,018. The alumina coating provides a barrier layer between the phosphor and the mercury-vapor and improves long term-UV maintenance.
Reflector tanning lamps (RTLs) are a special class of tanning lamps wherein the UV flux is focused by a reflector which causes the flux to be emitted over a narrower segment of the circumference, for example 140°, instead of through the entire 360° circumference of the lamp. Two key performance parameters of tanning lamps, and RTLs in particular, are the initial UVA output of the lamp and the UVA maintenance. The initial UVA output of the lamp (0 hours) will be herein referred to as the 0 h UVA output. The UVA maintenance measures the ability of the lamp to maintain its initial UVA output over a period of time, usually the first 100 hours of operation, and is expressed as a percentage of the initial output. As used herein, the 100 h UVA maintenance is defined as the ratio of the UVA output of the lamp after 100 hours of operation to its initial UVA output times 100% ((100 h UVA output/0 UVA output)×100%). For commercial applications, it is very desirable to have both a high 0 h UVA output and a high 100 h UVA maintenance. Unfortunately, current commercial RTLs either have a high 0 h UVA output (≧8600 μW/cm2) or a high 100 h UVA maintenance (≧87%) but not both attributes simultaneously. In other words, RTLs that have a high 100 h UVA maintenance (≧87%) have a 0 h UVA output that is much less than 8600 μW/cm2. Conversely, RTLs that have a high UVA output (≧8600 μW/cm2) have a 100 h UVA maintenance that is much less than 87%. The inverse relationship between 0 h UVA output and 100 h UVA maintenance for commercial reflector tanning lamps is shown in FIG. 1.
Therefore, it would be an advantage over the present state of the art to have tanning lamps, and RTLs in particular, that exhibit higher 0 h UVA outputs at high 100 h UVA maintenances.